Polylactic acid is a macromolecule which can be practically subjected to melt molding and, because of its biodegradable property, it has been developed as biodegradable plastics that are degraded after use under a natural environment to be released as carbon dioxide gas and water. In addition, since the raw material of polylactic acid itself is a renewable resource (biomass) originated from carbon dioxide and water, release of carbon dioxide after its use neither increases nor decreases carbon dioxide in the global environment. Such a carbon-neutral nature of polylactic acid is drawing attention in recent years, and it is expected to be used as an eco-friendly material. Further, lactic acid, which is the monomer for polylactic acid, can be inexpensively produced by fermentation methods using microorganisms in recent years, and polylactic acid is therefore being studied as a material alternative to general-purpose polymers made of petroleum-based plastics. However, in comparison with petroleum-based plastics, polylactic acid has lower heat resistance and durability, and also has lower productivity due to its lower crystallization rate. Therefore, its practical use is largely restricted at present.
As a means to solve such problems, use of a polylactic acid stereocomplex is drawing attention. A polylactic acid stereocomplex is formed by mixing optically active poly-L-lactic acid (hereinafter referred to as PLLA) and poly-D-lactic acid (hereinafter referred to as PDLA), and its melting point reaches 220° C., which is 50° C. higher than the melting point of a polylactic acid homopolymer, 170° C. Therefore, attempts are being made to apply polylactic acid stereocomplexes to production of fibers, films and resin molded articles having high melting points and high crystallinities.
Conventionally, a polylactic acid stereocomplex is formed by mixing PLLA and PDLA in the solution state or by melt mixing of PLLA and PDLA under heat. However, the method by mixing of PLLA and PDLA solutions requires evaporation of the solvent after the mixing and production process is therefore laborious, resulting in high cost of the polylactic acid stereocomplex, which is problematic. Further, in the cases of melt mixing of PLLA and PDLA under heat, these need to be mixed at a temperature that allows sufficient melting of the polylactic acid stereocomplex, but such a temperature also causes thermal degradation reaction of polylactic acid, leading to decreased physical properties of the molded article, which is problematic. Further, in cases where high-molecular-weight PLLA and high-molecular-weight PDLA are melt-mixed under heat, the melting points of the polylactic acid homopolymers do not disappear even with a mixing composition ratio of 50:50, so that it is currently impossible to obtain a material having both heat resistance and durability.
On the other hand, as techniques that enable formation of a stereocomplex even with high-molecular-weight polylactic acid, polylactic acid block copolymers composed of PLLA segments and PDLA segments have been disclosed (JP 2003-238672 A, JP 2006-28336 A, 3: JP 2006-307071 A and JP 2009-40997 A).
In JP 2003-238672 A, PLLA and PDLA prepared by ring-opening polymerization or direct polycondensation were melt-mixed under heat to prepare a mixture, which was then subjected to solid-phase polymerization, to obtain a polylactic acid block copolymer.
In JP 2006-28336 A, PLLA and PDLA obtained by melt polymerization were melt-mixed under heat, and the resulting mixture was subjected to solid-phase polymerization, to prepare a polylactic acid block copolymer.
In JP 2006-307071 A, PLLA and PDLA were mixed at a temperature close to the melting point and subjected to solid-phase polymerization in the presence of crystals of the polylactic acids alone, to prepare a polylactic acid block copolymer.
In JP 2009-40997 A, PLLA and PDLA obtained by direct polycondensation were mixed at a temperature higher than the melting point, and the resulting mixture was subjected to solid-phase polymerization, to obtain a polylactic acid block copolymer.
In the technique of JP 2003-238672 A, melt mixing needs heating to a temperature higher than the melting point of the polylactic acid stereocomplex, so that decrease in the molecular weight of the mixture during melt mixing is problematic. Further, because of requirement of prolonged reaction in solid-phase polymerization, improvement of the productivity has been demanded.
In the technique of JP 2006-28336 A, high-molecular-weight polylactic acid block copolymers can be obtained only in cases where the mixing composition ratio between PLLA and PDLA was apart from 50:50. In such cases, due to low stereocomplex formation, the obtained polylactic acid block copolymer is unlikely to have improved heat resistance and crystallinity, which is problematic.
In the technique of JP 2006-307071 A, formation of a stereocomplex is controlled only by the kneading temperature, and partial melting is found during kneading. Therefore, the crystal properties of the mixture are insufficient and also variable. Further, a polylactic acid block copolymer prepared by solid-phase polymerization of this kneaded product also has insufficient crystal properties, which is problematic.
In the technique of JP 2009-40997 A, since the molecular weights of PLLA and PDLA used for kneading are not more than 50,000, prolonged reaction is required for achieving a high molecular weight by solid-phase polymerization. Further, since the yield after the solid-phase polymerization needs to be increased, improvement of the productivity has been demanded.
It could therefore be helpful to provide a method for producing a polylactic acid block copolymer that forms a polylactic acid stereocomplex having a high molecular weight and a high melting point.